With tightening of regulation on exhaust gas of automobiles, reduction of the amount of nitrogen oxides (NOX) within exhaust gas has been demanded, and NOX sensors capable of directly measuring the concentration of NOX contained in exhaust gas have been developed.
Such an NOX sensor includes a gas sensor element including a plurality of cells each composed of an oxygen-ion-conductive solid electrolyte layer formed of, for example, zirconia, and a pair of electrodes formed thereon, and detects the concentration of NOX on the basis of an output from the gas sensor element. Further, the gas sensor element includes a heater for activating the solid electrolyte layer.
The NOX sensor has a structure in which a first pumping cell, an oxygen concentration detection cell, a second pumping cell, and a heater are stacked. Each of the cells is composed of a solid electrolyte layer and a pair of electrodes formed thereon. The concentration of oxygen within a first measurement chamber is measured by the oxygen concentration detection cell, and the supply of electricity to the first pumping cell is controlled such that a to-be-measured gas (exhaust gas) introduced into the first measurement chamber has a predetermined oxygen concentration. That is, the current flowing through the first pumping cell is controlled such that the output of the oxygen concentration detection cell assumes a preset value, whereby the oxygen concentration of the to-be-measured gas is adjusted (controlled). Further, the to-be-measured gas having a controlled oxygen concentration flows from the first measurement chamber to a second measurement chamber. Through application of a constant voltage to the second pumping cell, NOX contained in the to-be-measured gas is decomposed to N2 and O2. At that time, a second pump current which flows between the pair of electrodes of the second pumping cell is measured, and the concentration of NOX contained in the to-be-measured gas is detected from the measured second pump current.
Such an NOX sensor requires various control circuits, such as a control circuit for adjusting the current flowing through the first pumping cell, a control circuit for maintaining the inter-electrode voltage of the second pumping cell constant, a circuit for detecting the second pump current, and a control circuit for controlling the supply of electricity to the heater so as to adjust generation of heat. In general, a sensor control circuit unit in which these circuits are mounted on a single circuit board is used. However, such a sensor control circuit unit in which various control circuits are mounted on a single circuit board has a problem in that noise (an influence of an electromagnetic field) from a heater circuit for controlling the supply of electricity to a heater enters a signal detection circuit for detecting the second pump current. The heater circuit is apt to generate noise because of ON/OFF control of the current supplied to the heater.
In a technique proposed in order to solve such a problem, a reference potential section is provided between a heater circuit and an Ip2 cell (second pumping cell) detection circuit so as to electrically isolate these circuits, to thereby prevent entry of noise into the Ip2 cell detection circuit (see Patent Document 1).
In another proposed technique, a ground pattern of a sensing circuit (corresponding to the above-mentioned Ip2 cell detection circuit) and a ground pattern of a heater drive circuit are separately provided on the opposite sides of a ground terminal (see Patent Document 2). According to this technique, variation in the reference potential (ground) of the sensing circuit is prevented, whereby the reference potential becomes stable, and, thus, the influence of noise from the heater drive circuit can be mitigated.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2007-171024
[Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2004-212284